The invention relates to the field of reducing primary harmful substances formed in combustion units by means of a catalyst. The invention relates in particular to an exhaust gas purification system for denoxing exhaust gases of combustion units on a heatable and pressure-tight converter comprising an SCR catalyst, in which a substance, thermolytically separating NH.sub.3, or a corresponding substance mixture is included or an NH.sub.3 store for the intermediate storage of NH.sub.3 thermolytically separated from the substance or the substance mixture, a feed line connecting the converter with the exhaust gas for feeding the NH.sub.3 into the exhaust flow and a dosing device integrated into the feed line for dosing the NH.sub.3 quantity to be introduced into the exhaust flow. The invention further relates to a process for filling a converter of such an exhaust gas purification system with a substance thermolytically separating of an NH.sub.3 quantity, namely ammonium carbamate.
Such an exhaust gas purification system is known from DE 197 20 209 C1 of the applicant. With the exhaust purification system described in this document an NH.sub.3 supply into the exhaust gas line connected to a combustion unit is possible. For this purpose the exhaust gas purification system uses a heatable pressure-tight converter in which a substance thermolytically separating NH.sub.3, for example ammonium carbamate, is contained. By heating the converter, the ammonium carbamate breaks down into the two gases, NH.sub.3 and CO.sub.2. This gas mixture is introduced into an intermediate store which intermediate store can also be the converter itself. The converter is connected via a feed line to the exhaust gas line, via which the generated NH.sub.3 can be mixed into the exhaust flow. The feed line terminates in the exhaust gas line directly in front of an SCR (Selective Catalytic Reduction) catalyst In the feed line a clocked valve is provided as dosing device, with which the NH.sub.3 quantity to be introduced into the exhaust gas flow can be dosed. For this purpose, the clocked valve is controlled as a function of the acquired engine characteristics which permit conclusions regarding the NO.sub.x quantity contained in the exhaust gas flow.
The intermediate storage of a small quantity of NH.sub.3 can be classified as being harmless, and through the provided after-production it is ensured that there is always the capability of injecting the requisite quantity of NH.sub.3 into the exhaust gas flow such that as complete as possible a denoxing can be attained on the succeeding SCR catalyst. Use of a substance thermolytically separating NH.sub.3, such as for example ammonium carbamate, has the advantage, compared to handling the NH.sub.3 gas, of allowing handling without problems at normal ambient temperatures. Compared to the use of urea, the use of ammonium carbamate has the advantage that thermolytically separating NH.sub.3 in the necessary quantity starts already at approximately 60.degree. C. and is largely completed at 80 to 100.degree. C. such that the converter can be heated with a cooling system associated with an internal combustion engine.
In practice, the concept of denoxing realized in the prior known exhaust gas purification system has proven to be useful. The issue is nevertheless improving the known exhaust gas purification system with respect to some points by themselves or also in combination, which improvements relate in particular to the use of the exhaust gas purification system in a motor vehicle, for example, with a Diesel engine:
When using such an exhaust gas purification system in a motor vehicle it must be anticipated that the converter is ruptured in the event of an accident. This does not entail any problems if the converter is cold and under these conditions only the ammonium carbamate used is exuded. With very low vapor pressure a small portion of it will become diluted with the ambient air; the greatest amount can be collected. However, if the converter is in its heated operating state, a specific gas pressure exists in its interior with a corresponding NH.sub.3 quantity. One invention is therefore based on the task of providing a converter for an exhaust gas purification system according to the species, whose hazard potential in the event of an accident is decreased.
When starting the engine of a motor vehicle the converter is cold; only with increasing heating of the converter through increasingly hotter cooling water of the combustion engine does the converter become heated to its operating temperature for providing the requisite NH.sub.3 quantity as well as the necessary pressure. It therefore takes a specific length of time until the converter is operationally ready to provide the required NH.sub.3 quantity. However, it is desired that when starting an engine a sufficient quantity of NH.sub.3 is already available for injection into the exhaust gas flow. It is therefore a task of a further invention to develop the exhaust gas purification system further such that even at a cold start of a combustion engine the desired NH.sub.3 quantity is available.
The converter of the exhaust gas purification system according to the species is connected via a feed line to the exhaust gas line. The converter is usually disposed at a spacing from the exhaust gas line. It has been found that when using ammonium carbamate as the substance thermolytically separating NH.sub.3 on this section the NH.sub.3 --CO.sub.2 gas mixture in the feed line, in particular in the valves disposed therein, can form back again to ammonium carbamate in its solid form. In an unfavorable case, this can lead to an occlusion of the feed line. It is therefore a task of a further invention to provide an exhaust gas purification system according to the species, in whose feed line the crystallization of the substance thermolytically separating NH.sub.3 out of the gaseous phase carried in the feed line is prevented.
If the substance thermolytically separating NH.sub.3 in the converter, for example, ammonium carbamate is completely or substantially completely gasified, no more NH.sub.3 or no sufficient amount of NH.sub.3 can be produced. For this purpose it is provided to remove the converter from the system and to exchange it against a filled on. However, this entails the disadvantage that in this case corresponding connections in the feed line must be provided. The exchangeability further has a disadvantageous effect on the heating due to, for example, a heating coil encompassing the converter, since the heat transfer path in this case is relatively long and is discontinuous due to the gap between the converter and the heat coil. Further, it will be considered to dispose the converter in a motor vehicle at such a location that it is not destroyed in the event of an accident. In this case, the accessibility to the converter is occasionally not without problems. A further invention is therefore based on the task of providing an exhaust gas purification system according to the species in which a converter exchange, in order to make available fresh substance thermolytically separating NH.sub.3, is not necessary. Accordingly, this invention is also based on the task of providing such a filling process.